Transforming Growth Factor (TGF)-beta signals through a pathway that is simple yet has the potential for complexity and modulation at many levels. Alterations in TGF-beta signaling play an important role in cancer and metastasis. This is particularly true in the liver, which requires exquisite regulation of TGF-beta signaling for the maintenance of normal tissue homeostasis. One potentially important but poorly understood modulator of TGF-beta signaling is betaglycan, also known as the type III TGF-beta receptor. Betaglycan is an accessory receptor for TGF-beta that has both a full-length, membrane-anchored form and a proteolytically-cleaved soluble form. The function of betaglycan has traditionally been understood to be enhancement of TGF-beta signaling. Preliminary data demonstrate that it is also an inhibitory receptor, and that its glycosaminoglycan modifications play an important role in determining its function. Additional preliminary data suggests that the soluble form of betaglycan, as well as the membrane-anchored cytoplasmic domain remaining after cleavage of the ectodomain, are present at high levels and are potentially important regulators of TGF-beta signaling. The central hypothesis of this proposal is that betaglycan, acting through its full-length form and both proteolytically cleaved forms, is an important modulator of TGF-beta signaling in normal and malignant cells. Using human hepatoma cells in culture as a model system, we propose a series of experiments to demonstrate that betaglycan regulates TGF-beta signaling and to determine its mechanisms of action. Specifically, we propose to: 1. Characterize the interactions between betaglycan and the two serine-threonine kinase receptors, and determine how betaglycan mediates interactions between the kinase receptors; 2. Determine the regulation and function of the soluble receptor; 3. Determine the function of the transmembrane/cytoplasmic protein. These innovative approaches will provide new mechanistic insight into the important role of TGF-beta signaling in hepatocyte proliferation and malignant transformation. They will also greatly increase our understanding of TGF-beta signaling in general, and will pave the way for in vivo studies of beta glycan.